Description

The
fopen()
function opens the file whose name is the string pointed to by
path
and associates a stream with it.

The argument
mode
points to a string beginning with one of the following sequences
(possibly followed by additional characters, as described below):

r

Open text file for reading.
The stream is positioned at the beginning of the file.

r+

Open for reading and writing.
The stream is positioned at the beginning of the file.

w

Truncate file to zero length or create text file for writing.
The stream is positioned at the beginning of the file.

w+

Open for reading and writing.
The file is created if it does not exist, otherwise it is truncated.
The stream is positioned at the beginning of
the file.

a

Open for appending (writing at end of file).
The file is created if it does not exist.
The stream is positioned at the end of the file.

a+

Open for reading and appending (writing at end of file).
The file is created if it does not exist.
The initial file position for reading is at the beginning of the file,
but output is always appended to the end of the file.

The
mode
string can also include the letter aqbaq either as a last character or as
a character between the characters in any of the two-character strings
described above.
This is strictly for compatibility with C89
and has no effect; the aqbaq is ignored on all POSIX
conforming systems, including Linux.
(Other systems may treat text files and binary files differently,
and adding the aqbaq may be a good idea if you do I/O to a binary
file and expect that your program may be ported to non-UNIX
environments.)

Reads and writes may be intermixed on read/write streams in any order.
Note that ANSI C requires that a file positioning function intervene
between output and input, unless an input operation encounters end-of-file.
(If this condition is not met, then a read is allowed to return the
result of writes other than the most recent.)
Therefore it is good practice (and indeed sometimes necessary
under Linux) to put an
fseek(3)
or
fgetpos(3)
operation between write and read operations on such a stream.
This operation may be an apparent no-op
(as in fseek(..., 0L, SEEK_CUR)
called for its synchronizing side effect).

Opening a file in append mode (a as the first character of
mode)
causes all subsequent write operations to this stream to occur
at end-of-file, as if preceded the call:

fseek(stream, 0, SEEK_END);

The
fdopen()
function associates a stream with the existing file descriptor,
fd.
The
mode
of the stream (one of the values "r", "r+", "w", "w+", "a", "a+")
must be compatible with the mode of the file descriptor.
The file position indicator of the new stream is set to that
belonging to
fd,
and the error and end-of-file indicators are cleared.
Modes "w" or "w+" do not cause truncation of the file.
The file descriptor is not dup'ed, and will be closed when
the stream created by
fdopen()
is closed.
The result of applying
fdopen()
to a shared memory object is undefined.

The
freopen()
function opens the file whose name is the string pointed to by
path
and associates the stream pointed to by
stream
with it.
The original stream (if it exists) is closed.
The
mode
argument is used just as in the
fopen()
function.
The primary use of the
freopen()
function is to change the file associated with a standard text stream
(stderr, stdin, or stdout).

Return Value

Upon successful completion
fopen(),
fdopen()
and
freopen()
return a
FILE
pointer.
Otherwise, NULL is returned and
errno
is set to indicate the error.

Errors

EINVAL

The
mode
provided to
fopen(),
fdopen(),
or
freopen()
was invalid.

The
fopen(),
fdopen()
and
freopen()
functions may also fail and set
errno
for any of the errors specified for the routine
malloc(3).

The
fopen()
function may also fail and set
errno
for any of the errors specified for the routine
open(2).

The
fdopen()
function may also fail and set
errno
for any of the errors specified for the routine
fcntl(2).

The
freopen()
function may also fail and set
errno
for any of the errors specified for the routines
open(2),
fclose(3),
and
fflush(3).

Conforming To

The
fopen()
and
freopen()
functions conform to C89.
The
fdopen()
function conforms to POSIX.1-1990.

Notes

Glibc notes

The GNU C library allows the following extensions for the string specified in
mode:

c (since glibc 2.3.3)

Do not make the open operation,
or subsequent read and write operations,
thread cancellation points.
This flag is ignored for
fdopen().

e (since glibc 2.7)

Open the file with the
O_CLOEXEC
flag.
See
open(2)
for more information.
This flag is ignored for
fdopen().

m (since glibc 2.3)

Attempt to access the file using
mmap(2),
rather than I/O system calls
(read(2),
write(2)).
Currently,
use of
mmap(2)
is attempted only for a file opened for reading.

x

Open the file exclusively
(like the
O_EXCL
flag of
open(2)).
If the file already exists,
fopen()
fails, and sets
errno
to
EEXIST.
This flag is ignored for
fdopen().

In addition to the above characters,
fopen()
and
freopen()
support the following syntax
in
mode:

,ccs=string

The given
string
is taken as the name of a coded character set and
the stream is marked as wide-oriented.
Thereafter, internal conversion functions convert I/O
to and from the character set
string.
If the
,ccs=string
syntax is not specified,
then the wide-orientation of the stream is
determined by the first file operation.
If that operation is a wide-character operation,
the stream is marked wide-oriented,
and functions to convert to the coded character set are loaded.

Bugs

When parsing for individual flag characters in
mode
(i.e., the characters preceding the "ccs" specification),
the glibc implementation of
fopen()
and
freopen()
limits the number of characters examined in
mode
to 7 (or, in glibc versions before 2.14, to 6,
which was not enough to include possible specifications such as "rb+cmxe").
The current implementation of
fdopen()
parses at most 5 characters in
mode.

See Also

Colophon

This page is part of release 3.80 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
http://www.kernel.org/doc/man-pages/.

License & Copyright

Copyright (c) 1990, 1991 The Regents of the University of California.
All rights reserved.
This code is derived from software contributed to Berkeley by
Chris Torek and the American National Standards Committee X3,
on Information Processing Systems.
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@(#)fopen.3 6.8 (Berkeley) 6/29/91
Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
Modified, aeb, 960421, 970806
Modified, joey, aeb, 2002-01-03